The invention is related to a digital Acoustic Echo Control (AEC) unit of telephones. The purpose of the AEC is to prevent the far-end speaker's speech circulating back as an echo after coming out from the near-end phone user's loudspeaker and partly picked up by the phone's microphone. A general concept is illustrated in FIG. 1 where “i” denotes the sampling time index. Advanced AEC units contain an echo canceller module 21, generally consisting of an echo canceller 10 with a gradient adaptation means 12 and an adder 30, and a residual echo suppressor (sometimes referred to as a postfilter) 14 for residual echo suppression.
The need for an AEC unit in the hands-free telephones basically arises from an acoustic echo path with an impulse response g(i) from a local loudspeaker 16 to a local microphone 18. The objective of the echo canceller 10 with an impulse response c(i) is to find a replica of the echo path in order to compensate for an echo signal d(i) 22 of a voice signal x(i) 20 received by a loudspeaker 16 that provides an acoustic output signal in response to the voice signal x(i) 20, thus generating in the microphone 18 the echo signal d(i) 22 which is one of the components of a microphone signal y(i)=d(i)+s(i)+n(i) 28, where y(i) is a microphone speech signal and n(i) is a background noise signal. As the system identification process is always performed in the presence of observation noise (local speech plus background noise), s(i)+n(i), the objective of c(i)=g(i) cannot be reached exactly. The echo canceller 10 generates an estimate echo signal d′(i) 32 which is negatively added to the microphone signal 18 by the adder 30 which generates an echo reduced microphone signal e(i) 34 containing the partially compensated echo signal. The echo reduced microphone signal e(i) 34 is further provided to the gradient adaptation means 12 and to the residual echo suppressor 14. The gradient adaptation means 12 further provides a control signal 15 to the echo canceller 10 by determining a gradient of the controlled signal based on predetermined criteria using the voice signal x(i) 20 and the echo reduced microphone signal e(i) 34 as input signals. The purpose of the residual echo suppressor 14 is further reducing of residual echo components of the echo reduced microphone signal e(i) 34. The resulting output system signal s′(i)+n′(i) 36 after residual echo suppression by the residual echo suppressor 14 is then transmitted to the far speaker.
The basic principles of how to generate and control the echo canceller 10 and the residual echo suppressor 14 are well known. However, there are some problems involved in controlling them efficiently in a most optimal way. The key variable in the whole control issue is the residual echo, b(i)=d(i)−d′(i) which, unfortunately, cannot be directly determined since it is inherently embedded in the echo reduced microphone signal e(i)=b(i)+s(i)+n(i) 34.
The echo canceller module 21 of FIG. 1 often provides an insufficient estimate of the echo signal d(i) 22. The residual echo suppressor 14, in the sending path of the telephone, performs residual echo suppression, but in many solutions this is achieved at the cost of distortions (attenuations) of the useful signal s(i)+n(i). This is because the echo canceller 10 does the echo reduction in phase and magnitude, whereas, the residual echo suppressor 14 does it only in magnitude. In an alternative solution, the echo canceller module 21 can be used alone without a residual echo suppressor 14. This approach does not introduce noticeable signal distortions, but normally requires a very sophisticated control mechanism for the echo canceller. A more simple and effective approach is needed.